1. Field of the Invention
The present invention relates generally to rotary surgical instruments having an inner member rotatably disposed within an outer member such that a cutting element on the inner member rotates past an aperture in the outer member to cut anatomical tissue positioned in the aperture. More particularly, the invention pertains to rotary surgical instruments in which the cutting element of the inner member comprises a plurality of cutting teeth.
2. Brief Discussion of the Related Art
Rotary surgical instruments are commonly employed in various surgical procedures to cut anatomical tissue at an operative site. Rotary surgical instruments are especially advantageous for use in endoscopic or minimally invasive surgical procedures where access to the operative site is gained via a narrow or relatively small size portal or incision. Rotary surgical instruments typically comprise an elongate tubular outer member and an elongate inner member rotatably disposed within the outer member. A distal end of the tubular outer member ordinarily has a side-facing aperture or window providing communication with the internal lumen of the outer member. The inner member is rotatably disposed within the internal lumen of the outer member, and a distal end of the inner member has a cutting element that rotates past the aperture in the outer member to cut anatomical tissue positioned in the aperture. Proximal ends of the outer and inner members are conventionally attached to respective hubs for being coupled with a powered surgical handpiece. The outer member hub is ordinarily coupled with the handpiece to hold the outer member in a fixed or stationary position. The inner member hub is ordinarily coupled with the drive shaft of a motor of the handpiece, which rotates the inner member relative to and within the outer member to rotate the cutting element past the aperture. In many rotary surgical instruments, irrigating fluid is supplied to the operative site through an irrigation passage defined by an annular gap or clearance between the outer and inner members.
It is conventional for the inner member of rotary surgical instruments to be tubular and for the distal end of the inner member to have a side-facing aperture or window that provides communication with the internal lumen of the inner member. The aperture in the inner member repetitively comes into rotational alignment with the aperture in the outer member as the inner member rotates within the outer member. The internal lumen of the inner member may be coupled with a suction source to serve as a suction or aspiration passage providing suction or aspiration at the operative site via the rotationally aligned apertures to evacuate fluid and anatomical tissue.
The aperture in the inner member is normally circumscribed by a peripheral surface, and the cutting element of the inner member is commonly formed by a sharp cutting edge of the peripheral surface of the inner member aperture. Similarly, the aperture in the outer member is commonly circumscribed by a peripheral surface, and the outer member is typically provided with a cutting element formed by a sharp cutting edge of the peripheral surface of the outer member aperture to cooperate with the cutting edge of the inner member to cut anatomical tissue. In some rotary surgical instruments, the outer member cutting element includes two straight cutting edges, i.e. cutting edges without a toothed configuration, respectively extending longitudinally along opposite sides of the outer member aperture. The opposite sides of the outer member aperture may follow the path of an oblique or angled plane in profile as represented by U.S. Pat. No. 6,342,061 to Kauker et al and U.S. Pat. No. 5,383,884 to Summers. The opposite sides of the outer member aperture may follow an arcuate or curved path in profile as illustrated by U.S. Pat. No. 3,732,858 to Banko. The inner member cutting element oftentimes includes two straight cutting edges, i.e. cutting edges without a toothed configuration, respectively extending longitudinally along opposite sides of the inner member aperture. The opposite sides of the inner member aperture may similarly follow the path of an oblique or angled plane or may follow an arcuate or curved path in profile. Typically the cutting edges are formed by grinding the tubular member to obtain the requisite sharpness. Rotary surgical instruments of the latter type are generally known as a “full radius” design. Rotary surgical instruments that provide a more aggressive cutting action over a “full radius” design are those in which the cutting element of the outer member comprises two straight cutting edges as in the “full radius” design, while the cutting element of the inner member includes two rows of cutting teeth respectively extending longitudinally along the opposite sides of the inner member aperture. The most aggressive cutting action is generally obtained with rotary surgical instruments in which the cutting element of each of the outer and inner members includes two rows of cutting teeth respectively extending longitudinally along the opposite sides of its aperture as represented by U.S. Pat. No. 6,533,749 B1 to Mitusina et al.
In rotary surgical instruments where at least the inner member cutting element is comprised of two rows of cutting teeth, the peripheral surface of the inner member aperture is typically configured to form a distal side and a proximal side of each cutting tooth which define the geometric profile of the cutting tooth. In many cases, the cutting teeth are formed to have a flat or planar distal side and a flat or planar proximal side extending upwardly from a base or root of the cutting tooth to a tip of the cutting tooth. It is common for the distal and proximal sides of the cutting teeth to define a triangular geometric profile. The included angle defined between the distal and proximal sides of the cutting teeth is commonly in the range of 600 to 1000. Sometimes the cutting teeth are formed with sides that curve concavely from the root to the tip of the cutting tooth.
In cases where the inner member cutting element comprises two rows of cutting teeth, the distal sides of the cutting teeth of one row are usually the same as the distal sides of the cutting teeth of the opposite row. The proximal sides of the cutting teeth of one row are also usually the same as the proximal sides of the cutting teeth of the opposite row, such that the cutting teeth of one row have the same geometric profile as the cutting teeth of the opposite row. In addition, the tips of the cutting teeth of one row are normally aligned with the tips of the cutting teeth of the opposite row in a lateral direction perpendicular to the central longitudinal axis of the inner member. Accordingly, each row of cutting teeth provides the same cutting action, and the cutting action provided by one row of the cutting teeth is thusly duplicative of the cutting action provided by the opposite row of the cutting teeth.
The cutting teeth are normally formed by grinding the tubular inner member to create a sharp cutting edge, or knife edge, on the cutting teeth, which normally involves providing the cutting teeth with a positive relief angle. The tips of the cutting teeth usually result in fragile sharp points, and a certain amount of flat is usually required to be provided at the tips to impart some resistance to bending, breakage, deformation or distortion. However, the flat required at the tips of the cutting teeth reduces the biting action of the cutting teeth and thusly reduces their effectiveness in cutting anatomical tissue. Typically the cutting teeth increase in thickness between their tips and their roots, and the root regions of the cutting teeth provide little or no knife shearing cutting action on the anatomical tissue. Anatomical tissue is cut by the cutting teeth initially with a knife shearing action as the tips and cutting edges in the upper or tip regions of the cutting teeth shear the anatomical tissue. The anatomical tissue is then squeezed into the spaces between the lower or root regions of adjacent cutting teeth for the final cutting action, which is usually a combination of shear cutting and tearing but predominantly tearing. In rotary surgical instruments wherein the outer member cutting element also comprises two rows of cutting teeth, the outer member cutting teeth are ordinarily formed in a similar manner and have similar characteristics as the inner member cutting teeth.
Conventional rotary surgical instruments of the aforementioned types have various disadvantages. Rotary surgical instruments having a “full radius” design require high torque and force to cut anatomical tissue, and the shearing load on the cutting edges is distributed over a relatively small area as limited by the length of the straight cutting edges for a given size aperture. In rotary surgical instruments where at least the inner member cutting element comprises two rows of cutting teeth, the geometric profile of the teeth is significantly under-utilized for shear cutting since little or no shearing action takes place at the lower or root regions of the cutting teeth and most if not all of the shearing action takes place at the upper or tip regions of the cutting teeth. The shearing action is non-uniform or discontinuous because it is confined to and takes place over a limited or reduced portion of the geometric profile, and torque is also non-uniform or discontinuous. The limited shearing action reduces cutting effectiveness and results in the anatomical tissue being cut with greater trauma due to the squeezing and tearing of tissue that takes place in the root regions of the teeth. The fragility of the cutting teeth at their tips makes them susceptible to bending, deformation, distortion and breakage. Moreover, hoop stress within the tubular member in which the cutting teeth are formed increases the risk of distortion of the cutting teeth, particularly at the tips. Because the cutting teeth in one row have the same geometric profile, orientation and arrangement as the cutting teeth in the opposite row, their cutting action is duplicative which limits the aggressiveness of the cutting action.
It is desirable in rotary surgical instruments for the inner member to be rotatably disposed coaxially within the outer member. Oftentimes the inner member has a distal end wall in bearing contact with a distal end wall of the outer member to assist in maintaining coaxial alignment of the inner and outer members as the inner member rotates within the outer member. It is also generally desirable in rotary surgical instruments to minimize the external diameter of the outer member to reduce the size of the portal or incision needed for introduction of the instrument at the operative site. In many prior rotary surgical instruments, the geometric profile of the cutting teeth results in removal of mass or material from the tubular member above its central longitudinal axis in a large enough quantity to compromise proper axial alignment between the outer and inner members as well as proper bearing contact between the outer and inner members. Additionally, the potential for deformation arising from deficiencies in the strength and rigidity of the cutting teeth of prior rotary surgical instruments generally requires that there be greater annular clearance between the outer and inner members. Increased annular clearance between the outer and inner members usually necessitates that the external diameter of the outer member be increased, which may undesirably increase the size of the incision or portal needed for introduction of the rotary surgical instrument at the operative site. Furthermore, an increased annular clearance between the outer member and the inner member may make the inner member more susceptible to becoming axially misaligned with the outer member and may permit axial misalignments of greater magnitude.
Rotary surgical instruments that have cutting elements comprising a plurality of cutting teeth with concavely curved sides are represented by U.S. Pat. No. 5,833,692 to Cesarini et al, U.S. Pat. No. 6,217,598 B1 to Berman et al, U.S. Pat. No. 6,342,061 B1 to Kauker et al, and U.S. Pat. No. 6,419,684 B1 to Heisler et al. Cutting elements comprising a plurality of cutting teeth having concave sides as represented by the aforementioned patents generally result in removal of an even greater quantity of material or mass from the tubular member than triangular cutting teeth and are problematic for this and other reasons including fragility at the tips of the teeth, inadequate tooth strength and rigidity, limited shearing action, the potential for deformation, breakage, distortion, bending and misalignment, relatively high torque and force requirements to cut anatomical tissue, non-uniform or non-continuous shearing action and torque, and duplicative rows of cutting teeth on opposite sides of the aperture in the tubular member providing duplicative cutting action. U.S. Pat. No. 4,203,444 to Bonnell et al depicts a rotary surgical instrument in which the outer member has convexly curved formations without any tips. The convexly curved formations are not designed to cut anatomical tissue but, rather, cutting is effectuated by a helical cutting element on the inner member.
In rotary surgical instruments where the outer member is like the “full radius” design, the peripheral surface is commonly produced in the outer member along an oblique or angled plane extending through a side and distal end wall of the outer member. As a result, the peripheral surface circumscribing the aperture in the outer member has an inner peripheral edge adjoining the internal surface of the outer member and has an outer peripheral edge adjoining the external surface of the outer member. The peripheral surface has a width between the inner and outer peripheral edges and, therefore, between the internal and external surfaces of the outer member. The width of the outer member peripheral surface being defined by the full distance and cutting effectiveness of the outer member cutting element when used in combination with an inner member having a toothed cutting element.